1. Field of the Invention
This invention relates to motion compensation prediction systems for coding of interlaced video signals and, more particularly, to interlaced video signal motion compensation prediction systems, which are intended to improve the coding efficiency in block-by-block motion compensation prediction in interlaced video signal transmission or storage systems, and also to those for improving coding efficiency reduction due to incorporation of unnecessary motion vectors.
2. Description of the Prior Art
In high efficiency video signal encoding systems for the purposes of television conference video signal communication and storage of video signal in CD-ROMs, each picture frame or field is divided into blocks each of, for instance, 16 pels by 16 lines, and high efficiency coding is attained by using intra-picture coding or inter-picture coding of the difference between a reference and a pertinent present picture due to motion compensation.
FIG. 1 shows a usual encoder. Referring to the Figure, designated at 71 is a subtracter which derives the difference between an input and a predicted picture X1 and X2 to generate a prediction error picture X3. Designated at 72 is a coder, such as a discrete cosine transform (DCT) circuit or a vector quantizer, at 73 a quantizer, at 74 an inverse quantizer, and at 75 a decoder such as an inverse discrete cosine transform (IDCT) circuit or an inverse vector quantizer. Designated at 76 is an adder which adds a prediction error picture X5 restored in the decoder 75 and the predicted picture X2 to generate a local decoded picture X6. The local decoded picture X6 is used as the reference picture. As the reference picture, it is possible to use, in lieu of the local decoded picture X6, an original picture without being coded, i.e., a picture preceding or succeeding the input picture X1.
The local decoded and input pictures X6 and X1 are stored in a frame memory 77. A motion estimator 78 does block-by-block motion estimation. More specifically, it does motion estimation by receiving pertinent input block data 10 and reference block data 11 in a motion search region from the frame memory 77 and, after the motion estimation, outputs a motion vector ZV and a selection flag ZM. Using the motion vector ZV and selection flag ZM obtained from the motion estimator 78, a motion compensator 79 outputs the predicted picture X2 produced from the reference block data 11.
The output of the quantizer 73 is coded in a variable length code coder 80, and a multiplexer 81 multiplexes the output of the variable length code doder 80 with the motion vector ZV and selection flag ZM obtained from the motion estimator 78 to produce a coder information output.
FIG. 2 shows a prior art example of motion estimator 78. A frame motion estimator 84 estimates motion in a frame block and outputs a prediction error signal ER and a motion vector VR. A field motion estimator 85, on the other hand, estimates motion in a field block and outputs a prediction error signal EF and a motion vector VF. The prediction error signals ER and EF are compared in a comparator 87. The comparator 87 outputs, to a selector 86, a selection flag ZM for selecting the smaller one of the prediction error signals ER and EF. In response to this flag, the selector 86 selects the motion estimator providing the smaller one of the two prediction error signals and outputs the motion vector ZV from the selected motion estimator. As shown, in the prior art the prediction error signals are obtained from the frame and field motion estimators, and the motion vector from the motion estimator providing the smaller one of the prediction error signals is used for motion compensation, thus providing for improved coding efficiency.
In the above coder employing the motion estimator 78, in which motion estimation is done with respect to both the frame and field of a picture to select the motion estimator providing the smaller prediction error for the motion compensation, has the following problems.
(1) When an accelerated motion is presented in video, the reference picture for an even field of the input picture is different from that for the odd field since motion vectors for those fields are different. In this case, therefore, if the frame motion estimator 84 is selected, the prediction error in the motion compensation is increased to reduce the coding efficiency.
(2) With constant speed motion in video, the extent of motion in each field is substantially the same. This means that when the field motion estimator 85 is selected, information about the quantity of motion vectors is doubled compared to the case of the frame motion compensation and the coding efficiency reduces.
FIG. 3 shows a different prior art example of the motion estimator 78. The input block data 10 which is part of the frame or field of a picture and the reference block data 11 are coupled to 1, 2 . . . , n vector motion estimators 91, 92, . . . 9n to derive respective prediction error signals E1, E2 . . . En which are output to a comparator 101. The comparator 101 compares the prediction error signals E1 to En and outputs the selection flag ZM of the motion estimator providing the smallest prediction error signal to a selector 102. According to this selection signal ZM the selector 102 selectively outputs the motion vector ZV output from the motion estimator providing the smallest prediction error signal. In the case of n (n=2, 3 . . . ) vector motion estimations, n times the number of motion vectors are necessary compared to the case of 1 vector motion estimation.
In the prior art coder, the selected motion vector ZV is used for the motion compensation to provide for improved coding efficiency.
However, such a prior art coder has the following problem.
When the prediction error provided from each of the 1 to n vector motion estimators is not so large so that the prediction efficiency difference is small with the prediction error from any vector motion estimator while the prediction error provided from the n vector motion estimator is smallest, the n vector motion compensation is selected. Consequently, an increased number of motion vectors are sent out to reduce the coding efficiency.